1. Field of the Invention
The present invention relates to a steering damper used for damping an impact when a reverse input is applied from a steering wheel side such as a kickback or the like, in a power steering apparatus which operates a power cylinder so as to obtain a power assist force (a steering assist force) by switching and controlling a flow passage control valve in accordance with a steering operation of a steering handle.
2. Description of the Related Art
In general, in a hydraulic power steering apparatus, there is provided a flow passage control valve for selectively connecting a pump and a tank to left and right chambers of a power cylinder in accordance with a steering operation of a steering handle. A pressurized oil is supplied to any of the chambers of the power cylinder by the flow passage control valve in accordance with a steering operation so as to apply a power assist force for turning a steered wheel.
In an automobile mounting the power steering apparatus mentioned above thereon, there is proposed a structure in which a steering damper is provided in the power steering apparatus, for example, in Japanese Patent Application Laid-Open Publication Nos. 61-12468 and 62-43367.
The steering damper mentioned above is structured so as to damp and reduce an impact generated by a reverse input applied from a steering wheel side, for example, due to an unevenness on a running road surface, an obstacle or the like at a time when the automobile runs (so-called kickback; hereinafter, simply refer to a reverse input), thereby preventing the impact from being transmitted to the steering handle.
A most popular structure in the conventional steering damper is provided with a throttle or a variable throttle for applying a resistance to a pressurized oil flow discharged from a return side chamber of the power cylinder together with a movement of a piston when the reverse input mentioned above is applied to right and left cylinder passages between a flow passage control valve in the power steering apparatus and right and left chambers in the power cylinder.
However, since the throttle or the variable throttle mentioned above becomes a flow passage resistance against the pressurized oil flow from the pump and the return side flow to the tank at a time of a positive input together with the steering operation of the steering handle, the throttle or the variable throttle reduces a response at a time of operating the power cylinder.
Accordingly, there is proposed by Japanese Patent Application Laid-Open Publication No. 62-43367 mentioned above a structure which can sufficiently supply the pressurized oil to the cylinder chamber from the pump via the flow passage control valve by constructing the throttle mentioned above by a variable throttle valve, disposing check valves in parallel to the variable throttle valve in the right and left cylinder passages, and opening the check valve in the cylinder passage corresponding to the positive input time caused by the steering operation.
In this conventional art, the variable throttle valve is structured such that an opening degree thereof can be varied in dependence on a vibration frequency within an oil chamber provided in the steering damper main body.
However, since the return side flow from the cylinder chamber passes through the throttle or the variable throttle even in the case that the check valve mentioned above is provided, the flow passage resistance is generated and a response of the power cylinder is reduced.
Further, in the conventional steering damper mentioned above, since the variable throttle valve for damping the impact at the reverse input time from the steering wheel side and the return side flow oil passage at the positive input time together with the steering operation from the steering handle are the same, there is a case that the supply of the pressurized oil is prevented even if it is necessary to sufficiently supply the pressurized oil, for example, at a time of suddenly steering or the like.
That is, when the variable throttle valve is provided with attaching great importance to a performance of damping the impact at the reverse input time, it is impossible to obtain a sufficient return side flow from the cylinder chamber at the positive input time together with the steering operation, so that it is impossible to supply a necessary amount of pressurized oil to the corresponding cylinder chamber.
Further, when the pressurized oil is insufficiently supplied as mentioned above, an operation of the power cylinder is delayed due to a lack of the pressurized oil supply, so that a turn following performance of the steering wheel becomes insufficient. In particular, a so-called hooking is generated in the operation of the steering handle at the sudden steering time and there is a risk that the steering operation becomes non-smooth.
In the power steering apparatus, in order to enable a smooth steering handle operation with no hooking at the positive input time and improve a response of the power cylinder so as to improve a turn following performance, it is necessary to increase the passage diameters of the right and left passages reaching the right and left chambers of the power cylinder. However, when the structure is made in the manner mentioned above, there is generated a problem at the reverse input time from the steering wheel side such as the kickback mentioned above or the like, so that it is desirable to employ a countermeasure which can satisfy requirements opposing to each other.
Further, in the power steering apparatus, it is desirable to employ a countermeasure which can reduce the number of parts constituting the steering damper mentioned above, simplify the structure and be easily assembled, thereby achieving a compact size of a whole of the apparatus and reducing a cost.
The present invention is made by taking the above matters into consideration, and an object of the present invention is to provide a steering damper which can satisfy a function as a steering damper for reducing an impact at a reverse input time from a steering wheel side such as a kickback or the like and can improve a turn following performance at a positive input time together with a steering operation of a steering handle.
Further, another object of the present invention is to provide a steering damper which can reduce a number of parts, can simplify a structure, can be easily assembled, can make a whole of the apparatus compact and can reduce a cost.
In order to achieve the objects mentioned above, in accordance with a first aspect of the present invention, there is provided a steering damper attached to a power steering apparatus provided with a flow passage control valve for selectively switching and connecting among a pump, a tank and right and left chambers in a power cylinder in correspondence to a steering operation, and a pair of right and left cylinder passages for connecting the flow passage control valve to the right and left chambers in the power cylinder, comprising:
a pair of right and left damper portions having a variable throttle valve arranged in the middle of each of the cylinder passages and limiting an inflow to the flow passage control valve from the power cylinder and a pilot operated check valve connected to the variable throttle valve in parallel and allowing an inflow from the flow passage control valve to the power cylinder; and
a pilot plunger arranging the right and left damper portions at positions close to the respective cylinder passages and sliding between the right and left damper portions,
wherein respective end portions of the pilot plunger are set to a pressure receiving surface for receiving the respective fluid pressure by being faced within the right and left cylinder passages and an operating surface for opening the pilot operated check valve in another cylinder passage by being operated by the fluid pressure within one cylinder passage.
In accordance with a second aspect of the present invention, there is provided a steering damper as recited in the first aspect, wherein a minimum valve opening degree of the variable throttle valve is set so as to allow a flow of the pressurized fluid when a flow amount within the cylinder passage is a little and apply a resistance to a flow of the pressurized fluid when the flow amount within the cylinder passage is increased.
In accordance with a third aspect of the present invention, there is provided a steering damper as recited in the first aspect or the second aspect, wherein the pilot operated check valve is constituted by a ball and a cylindrical seat portion, and the variable throttle valve is constituted by a hole portion formed from one end of the cylindrical seat portion to another end thereof and a valve plate opening and closing another end of the hole portion.
In accordance with a fourth aspect of the present invention, there is provided a steering damper as recited in the first aspect or the second aspect, wherein both of the pilot operated check valve and the variable throttle valve are constituted by a valve in which a leaf spring is set to a valve body.
In accordance with a fifth aspect of the present invention, there is provided a steering damper as recited in the fourth aspect, wherein the leaf spring constituting the pilot operated check valve and the leaf spring constituting the variable throttle valve are overlapped with each other in a state that they are opened in opposite directions to each other.
In accordance with the present invention, the impact at the reverse input time from the steering wheel side is damped and reduced by the resistance generated by the return side flow from the return side chamber of the power cylinder passing through the variable throttle valve provided in the cylinder passage in the return side. At this time, a fluid pressure in the cylinder chamber side becomes greater than a fluid pressure in the flow passage control valve side of the pilot operated check valve in the return side cylinder passage, the return side pilot operated check valve maintains a closed state, and the return side flow is restricted. Further, the pilot plunger is moved so as to open the supply side pilot operated check valve, and the flow within the supply side cylinder passage is secured.
Further, in accordance with the present invention, at the positive input time together with the steering operation, it is possible to supply the pressurized oil from the pump to the cylinder chamber by opening the pilot operated check valve in the supply side cylinder passage. Further, it is possible to open the pilot operated check valve in the return side cylinder passage by the operating surface of the pilot plunger moving by receiving the fluid pressure within the supply side cylinder passage mentioned above on a pressure receiving surface so as to reduce the flow passage resistance within the return side cylinder passage, thereby securing the flow from the return side cylinder chamber.